Replace the global-exports mechanism with a caller-vmctx mechanism. (#789)

* Replace the global-exports mechanism with a caller-vmctx mechanism.

This eliminates the global exports mechanism, and instead adds a
caller-vmctx argument to wasm functions so that WASI can obtain the
memory and other things from the caller rather than looking them up in a
global registry.

This replaces #390.

* Fixup some merge conflicts

* Rustfmt

* Ensure VMContext is aligned to 16 bytes

With the removal of `global_exports` it "just so happens" that this
isn't happening naturally any more.

* Fixup some bugs with double vmctx in wasmtime crate

* Trampoline stub needed adjusting
* Use pointer type instead of always using I64 for caller vmctx
* Don't store `ir::Signature` in `Func` since we don't know the pointer
  size at creation time.
* Skip the first 2 arguments in IR signatures since that's the two vmctx
  parameters.

* Update cranelift to 0.56.0

* Handle more merge conflicts

* Rustfmt

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

tests/instantiate.rs

@@ -1,8 +1,5 @@
 use more_asserts::assert_gt;
-use std::cell::RefCell;
-use std::collections::HashMap;
 use std::path::PathBuf;
-use std::rc::Rc;
 use wasmtime_environ::settings;
 use wasmtime_environ::settings::Configurable;
 use wasmtime_jit::{instantiate, native, CompilationStrategy, Compiler, NullResolver};
@@ -24,14 +21,6 @@ fn test_environ_translate() {
 
     let mut resolver = NullResolver {};
     let mut compiler = Compiler::new(isa, CompilationStrategy::Auto);
-    let global_exports = Rc::new(RefCell::new(HashMap::new()));
-    let instance = instantiate(
-        &mut compiler,
-        &data,
-        None,
-        &mut resolver,
-        global_exports,
-        false,
-    );
+    let instance = instantiate(&mut compiler, &data, None, &mut resolver, false);
     assert!(instance.is_ok());
 }